FIG. 1(a) shows a typical adjustable speed drive configuration, where a bridge inverter consisting of six switching devices is used to supply an adjustable voltage of controlled frequency to a three phase Permanent Magnet (PM) motor. For PM motors having sinusoidal back electromotive force (EMF), a sinusoidal current is required to best mach the back EMF. A rectangular wave current is required for brushless direct current (BLDC) motors that have trapezoidal back EMFs. The actual current that the inverter can produce by means of pulse width modulation (PWM) contains harmonic components in addition to the required frequency component due to IGBT's limited switching frequency. The harmonic components in the motor currents appear as ripples and increase as the motor inductance decreases. The current ripple may become unacceptably large as the motor inductance decreases below certain values. The high current ripple also causes additional copper and iron losses in the motor. Other semiconductor devices such as metal-oxide semiconductor field effect transistor (MOSFET) that are capable of switching at higher frequency can not meet the voltage and/or current requirements for the high power PM motors (greater than 10 kWs).
One traditional and obvious way to limit current ripple is to add external inductance as shown in FIG. 1(b). For BLDC motor drives a quasi-current source inverter was proposed, as shown in FIG. 1(c), in which a step-down chopper consisting of a switch, a diode and an inductor is inserted into the DC bus. The chopper regulates the motor current by pulse width modulation (PWM) while the bridge inverter operates in a six-step manner with each device conducting a fixed period of 120 electrical degrees. This topology can reduce the number of external inductors as compared to the previous one. Other existing ways include the series or parallel connection of multiple modular inverters through transformer or inductor coupling, in which special arrangements of transformer windings and/or phase displacement in PWM carriers between the modular inverters are used to eliminate the lower order harmonic components, thus reducing the current ripple and effectively increasing the switching frequency. Disadvantages of adding inductors or transformers include that they are bulky, heavy and require large additional installation space.
These problems can be overcome in this invention by increasing the inverter effective switching frequency while maintaining each individual device switching within its limited capability.